Spinal fusion is a common technique used to treat chronic back pain caused by degenerated or herniated disk. The technique involves the removal of a disc between two vertebrae and replacing it with an intervertebral spacer. The intervertebral spacer maintains spacing between the two vertebrae and preferably results in fusion through the spacer. The intervertebral spacers may be constructed of autogenic bone tissue taken from a patient's own bone. Allogenic spacers are constructed of bone harvested from donors. Artificial spacers are currently the most common spacer type and may be constructed of metallic material such as titanium or stainless steel or polymers such as polyetheretherketone (PEEK).
PEEK has recently become popular due to its biocompatibility and naturally radiotranslucent characteristics, resulting in limited interference with x-ray and CT imaging. However, while PEEK is biocompatible, bone treats it as a foreign body during the remodeling process and isolates it with a fibrous tissue capsule. This fibrous tissue prevents direct bony apposition and adhesion to the implant. Other materials, such as titanium, allow for direct bony apposition and ongrowth, but they are typically not radiotranslucent and it becomes difficult to assess the fusion formation.
Other areas where PEEK is used as an orthopedic biomaterial experience similar fibrous encapsulation. Such indications include custom machined bodies that are used to fill defects in the skull and cranium. With PEEK, MRI and CT imaging is generally easier as compared to titanium, but the implant is never fully incorporated into the bone and soft tissue does not adhere to the implant.
Ceramic materials such as calcium phosphates, β-TCP, hydroxyapatites and the like allow for direct bony apposition much like titanium. However, they are typically limited in their strength and toughness. Therefore, it is desireable to construct a material that combines more of the desired properties from other individual materials described above, such as toughness and strength, less interference with MRI, X-ray or CT imaging, tissue adhesion, etc.